The present invention relates to a process for the production of a composite of a highly elastic material and a foam.
The initiation of chemical reactions by plasmas has been known for some time. (See, for example, H.V. Boenig, Plasma Science and Technology, Carl Hanser Verlag, Munich/Vienna (1982). The interactions between plasmas and substrate surfaces are manifold and complex. Depending upon the gas used, the substrate surface may be cleaved or etched by reactive atoms, a technique widely used in semiconductor manufacture (See, for example, M. Verzugopalan et al : Plasma Chemistry III, Topics in Current Chemistry 94, Springer Verlag, Berlin (1980). It is known that intensive UV irradiation occurs in the plasma. In certain polymers, this unwanted UV radiation can lead to crosslinking reactions at or on the substrate surface. The higher the degree of crosslinking, the more sensitive a material is to UV radiation. It is also known that, due to the unsaturated --C.dbd.C-double bonds, highly elastic materials based on rubbers such as polybutadienes, polyisoprenes, etc., are more UV-sensitive than non-elastic polymers such as polyethylenes, polypropylenes and polyacrylonitriles.
It is also known that any increase in the degree of crosslinking and hence in the density of the polymer surface is accompanied by a reduction in the adhesion of coatings subsequently applied to that polymer.
The foaming of high-adhesion polyurethane coatings onto elastic shoe soles is a problem which has still not been solved.
To apply firmly adhering, foamed polyurethane layers, the rubber surface has hitherto had to be degreased and then subjected to wet-chemical etching with environment-polluting oxidizing agents. An adhesive layer which had to be activated by heat before application of polyurethane (PU) foam layer was then applied. Wet-chemical treatment of the rubber surface with oxidizing agents has the further disadvantage that it leads to the formation of cracks and vacuoles in the rubber surface which adversely affect the mechanical properties and long-term stability of the PU-to-rubber bond.
It is known from the literature that aqueous oxidizing agents used, for example, for the pretreatment of ABS (acrylonitrile, butadiene and styrene) copolymers corrode the elastic polybutadiene particles so that cavities and vacuoles are formed in the ABS surface. The coatings of metals or plastics applied in the second step are firmly anchored onto these vacuoles. In these systems, the adhesion obtained is explained by the pushbutton theory (See, for example, R. Weiner, Kunststoff-Galvanisierung, Eugen G. Leuze Verlag, Saulgau/Wurtt (1973).
The above-mentioned adhesion mechanism is also relevant to the adhesion of the polyurethane coatings to the pre-etched rubber surface. The vacuoles and cavities produced by wet-chemical etching have an average size of several .mu.m and may be observed by means of a scanning electron microscope (x200 magnification).